One typical circuit used in this context is a linear charger circuit for a rechargeable battery. In such a case an adaptor provides a transformed output from the AC mains supply which provides a supply voltage via a rectifier to the DC-DC power supply of the computer. In addition, the output charges a rechargeable battery via a diode and transistor, and an output from the battery is supplied via a rectifier to the DC-DC power supply in the event that the mains supply is inoperative.
Another typical circuit uses a switching charger for the battery. However, rectifiers for the transformed AC mains supply and for the battery output are still required.
The power passing to the DC-DC power supply whether from the mains if this is operative or from the battery if this is supplying power, will in either case pass through a rectifier and so these need to be efficient to reduce heat generation and voltage drop. Generally they are therefore schottky rectifiers. Such rectifiers are relatively expensive, however, and still have a forward drop of around 500 millivolts, which if one assumes a typical current of 4 amps means a loss of 2 Watts which appears as heat.
This has serious disadvantages particularly when operating with battery power since this reduces the possible operating time on the internal batteries or alternatively forces the designer to use larger batteries which are a significant part of the overall weight and size of a compact portable personal computer. In addition a low efficiency equates with a higher heat output with the result that the loss appears as heat which, in a confined space, is not easy to dissipate. In particular, unlike normal desk-top personal computers the use of a ventilating fan or the like, would represent too significant a drain on the battery power. Therefore the power supply components cannot be packed closely together and the power supply must be of relatively large volume.
It is desirable therefore to reduce such losses and to achieve further improvement in the efficiency of the portable personal computer power supply.